Saturday, April 18, 2020

Interru(o)ptions

This post is just to document and explore some of the interrupt options on the CMS 9619 that could be compatible with NitrOS-9's interrupt requirements (60Hz based on VSYNC).

RTC-58323 Real Time Clock

The CMS 9619's RTC can output a standard clock signal. The CMS 9619 has jumper block (TS10) that allows you to select a signal to connect to the Control B input (CB1) on PIA0. The signals are:

  • 1 hour
  • 1 minute
  • 1 second
  • 1024 Hz

1024Hz seems too fast and 1Hz is too slow for the interrupt we need. But, it's nice to know it is there. Probably better for a watchdog timer.

MC6840 Programmable Timer Module

The MC6840 timer in the CMS 9619 can generate a continuous square wave from 1MHz (1/2 the system clock speed) to about 141 years. The MC6840 can generate an interrupt every low transition of the output, or any of outputs can be jumped to the Control A input (CA1) on PIA0 using the CMS 9619's TS14 jumper block. I am not sure why there are 2 options to generate the interrupt here though.

To get an approximately 60Hz signal, I need to configure the MC6840 with a $4000 (16,382) divider of the system clock (1/((16,382+1)(2E-6)*2) = 61.03Hz). This should be easy to configure in the MC6840 16-bit Continuous Operating Mode.

Using the MC6840 PTM with a jumper to the MC6821 Peripheral interface Adapter's CA1 seems like the most suitable choice. It is similar to NitrOS-9's standard VSYNC interrupt on the CoCo's PIAO CB1 so I can re-use most of the code, and seems highly configurable.

External Circuit

The MC6840 Application Guide contains a circuit for getting a 60Hz TTL signal from the 120V power lines. I think this signal could work as a trigger to the MC6840 or the MC6821 PIA. However, it is overly complicated for this project, which does not need exact long-term timing.

Aside

I spent some time watching the "Virtual" CoCoFEST for inspiration today. Check it out!

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